Tuning indicator circuit



July 5, 1938.

TUNING INDICATOR CIRCUIT Filed July 23, 1936 A. F. AMPLIFIER Z'I DET INVENTOR WOLFGANG F. EWALD ATTORNEY w. F. EWALD 2,122,562

Patented July 1938 UNITED. STATES PATENT OFFICE TUNING INDICATOR CIRCUIT tion of Germany Application July 23, 1936, Serial No. 92,066 In Germany July 26, 1935 7 Claims.

It is known that difiiculties are encountered in attempting an accurate tuning of a receiver to a carrier wave especially in receivers with fading control, in which owing to the control performance the volume does not decrease when tuning to the side bands, while incomplete tuning is noted only by a distortion of the tone. The customary optical indicating instruments have the disadvantage that they do not always afford an easy tuning to the optimum, the more so since it is necessary simultaneously to view the station dial as well as the tuning instrument. Devices have already been proposed to render the receiver inoperative automatically at inexact tuning; for instance, by means of a circuit with very steep characteristic and placed in parallel to the intermediate frequnecy part, which supplies, when resonance occurs, to a subsequently connected tube that is normally blocked, a counter potential by which the blocking is released, and thereby reproduction made possible. However, the means required for this device are considerable, so that this device could hitherto not be put to practice.

In accordance with the present invention it is proposed to produce either a constant, or variable steady, note, which is silenced only when the receiver is exactly tuned to the carrier wave. Hence at incorrect tuning to any carrier wave the tone is heard in the manner of a busy signal, but which disappears at the correct tuning to any desired transmitter, thus preventing the bearer from being unaware of the false tuning.

The subject matter of the invention will now be elucidated with reference to the circuit dia- 35 gram shown in the figure by Way of example. A

conventional superheterodyne receiver is shown,

the second detector supplying AVC bias. The

additional elements consist, for instance, of a tube a serving as oscillator, and having a grid 40 circuit a tuned to the intermediate frequency of the superheterodyne receiver. At the tuning of the receiver, a beat note is produced between the oscillations of the oscillator a and the intermediate frequency energy, whereby this note becomes the lower the more the tuning approaches the carrier wave of a transmitter. At exact tuning to the transmitter, the intermediate frequency will attain its required value and hence exact conformity to the frequency of the oscillator a, so that the beat note disappears.

Since, however, at small voltage variations, differences between the two frequencies, although they may be small, and hence beating can possibly not be avoided, it may be necessary to render the oscillator 11 inoperative at sharp tuning to the carrier wave. This is accomplished in that a corresponding bias is applied to one of the grids f of the oscillator, and this is derived from the intermediate frequency part of the receiver. The bias is such that it suffices only for rendering the oscillator inoperative, at tuning to the carrier wave. To facilitate this, a control resistor r is inserted in the cathode lead of the oscillator 11 and by means of which it is possible to adjust at will the oscillation tendency of this tube.

Obviously, the bias applied to the grid of the tube a used for blocking, depends upon the amplitude of the tuned carrier wave. However, especially when using fading control, it is possible immediately to adapt the arrangement in such manner that also in case of weak carrier Waves, the oscillator will be disconnected without causing a premature disconnection at strong carrier waves.

It will not always be appropriate to have the oscillator a proper oscillating on the intermediate frequency. As a rule it will rather be of advantage to utilize a fraction or a multiple of this frequency, and to utilize the upper harmonics of the fundamental oscillation for the production of the note.

What is claimed is:

1. In combination with a wave transmission system, means for detecting waves transmitted through the system, means for impressing waves of the same frequency as the transmitted wave frequency upon said detecting means so as to provide beat energy when said transmitted waves change in frequency from the impressed Wave frequency value, means for deriving a voltage from the transmitted waves whose magnitude varies directly with transmitted wave amplitude, and additional means for controlling said impressing means with said derived voltage in a sense to render the impressing means ineffective when said derived voltage is a maximum.

2. In combination with a wave transmission system, means for detecting waves transmitted through the system, means for impressing waves of the same frequency as the transmitted wave frequency upon said detecting means so as to provide beat energy when said transmitted waves change in frequency from the impressed wave frequency value, means for deriving a voltage from the transmitted waves whose magnitude varies directly with transmitted wave amplitude, additional means for controlling said impressing means with said derived voltage in a sense to render the impressing means ineffective when said derived voltage is a maximum, and means for controlling the wave transmission efiiciency to said detecting means with said derived voltage in a sense to maintain the wave amplitude at the latter uniform.

3., In a superheterodyne receiver of the type including an intermediate frequency amplifier, a second detector and an audio output network, the method of indicating accurate tuning of the receiver which includes producing local oscillations of a frequency equal to said intermediate frequency, feeding said oscillations to the second detector, deriving a unidirectional voltage from the intermediate frequency energy, and employing said derived voltage to prevent said oscillation production when said intermediate energy is of a frequency equal to the local oscillation frequency.

4. In a radio receiver which includes a signal amplifier and a detector whose input circuit is coupled thereto, means for tuning the amplifier to different stations in a tuning range, an accuracy tuning indicator device comprising a local oscillator circuit connected to produce oscillations of a frequency equal to the frequency of the signal energy impressed on the detector input circuit, means impressing said oscillations on said detector input circuit whereby a beat note is produced by the detector when the detector input energy differs in frequency from said oscillation frequency, an automatic'volume control circuit connected between said detector input circuit and amplifier, and a connection between the control circuit and said local oscillator for rendering the latter ineffective at the desired station settings of the tuning means.

5. In a superheterodyne receiver of the type including an intermediate frequency energy input circuit, a second detector tube and a local oscillator tube operating at the frequency of the intermediate frequency energy, means for impressing the local oscillations upon a point between the input circuit and the detector tube, and means responsive automatically to an increase in amplitude of said intermediate energy at resonance, for rendering the oscillator tube inoperative.

6. In a superheterodyne receiver, at least one intermediate frequency amplifier, a second detector, a network tuned to the intermediate fre-- quency coupling the amplifier output electrodes to the detector, a tube connected to produce oscillations of said frequency, means for impressing the oscillations on a point of the network, a circuit deriving a direct current voltage from intermediate frequency energy which varies directly in amplitude with the latter, and means for applying the voltage to an electrode of the oscillator tube in a sense to render the same inoperative.

7. In a superheterodyne receiver, at least one intermediate frequency amplifier, a second detector, a network tuned to the intermediate frequency coupling the amplifier output electrodes to the detector, a tube connected to produce oscillations of said frequency, means for impressing the oscillations on a point of the network, a circuit deriving a direct current voltage from intermediate frequency energy which varies directly in amplitude with the latter, means for' applying the voltage to an electrode of the oscillator tube in a sense to render the same inoperative, and additional means for applying the direct current voltage to an electrode of the amplifier in a sense to reduce the gain thereof.

WOLFGANG F. EWALD. 

